1. Field of the Invention
The embodiments discussed herein are directed to a head control method, a control device, and a storage device for controlling, by applying an energization amount to a heater element and thermally expanding a head element, a position of the head element that is opposed to a storage medium and can read out a signal, and, more particularly to a head control method, a control device, and a storage device that can accurately control, for respective heads, an energization amount applied to a heater element in any temperature environment and maintain a head element in a desired target flying height.
2. Description of the Related Art
Conventionally, as a method of controlling a clearance amount between a head element in a magnetic disk device and the surface of a magnetic disk (hereinafter referred to as a “flying height”), there is known a method of controlling a thermal expansion amount of the head element by changing an energization amount applied to a heater element in a head.
FIG. 18 is a diagram for explaining a method in the past for controlling a flying height. In an example shown in FIG. 18, before an energization amount is applied to a heater element, a flying height of a head 12a is lower than a flying height of a head 12b. Such a difference between the flying height is caused by individual variation in magnetic disk devices. In this case, as shown in FIG. 18, in order to set the flying height of the heads 12a and 12b to a target flying height, an energization amount applied to the heater element is controlled. Specifically, since a thermal expansion amount of a head element increases as an energization amount applied to the heater element is raised, an energization amount applied to the head 12b is controlled to be larger than an energization amount applied to the head 12a. 
The control of a flying height is required to be highly accurate control with an extremely small error with respect to the target flying height. This is because, when an error occurs in the control of a flying height, a probability of collision of the head element and the surface of the magnetic disk increases and, because of occurrence of thermal asperity, a head output attenuates and head noise increases. In particular, in recent years, a flying height is designed to be extremely small due to an increase in magnetic storage density of a magnetic disk. Therefore, control of the flying height must be more highly accurate.
As described above, since the head element thermally expands, an amount of thermal expansion of the head element is different depending on the temperature (environmental temperature) in the magnetic disk device. In other words, even if the same energization amount is applied to the heater elements, the thermal expansion amount of the head element increases as the environmental temperature rises and the flying height becomes smaller than an intended flying height. Therefore, it is difficult to highly accurately control the flying height of the head element. Under a situation in which highly accurate control of a flying height is required, it is important to control a flying height taking into account the environmental temperature.
Therefore, several techniques for controlling a flying height taking into account environmental temperature have been proposed. For example, Japanese Patent Laid-Open No. 2006-164388 (hereinafter, Patent Document 1) discloses a technique for controlling an energization amount applied to a heater element according to environmental temperature measured by a temperature sensor (a thermistor, etc.) in a magnetic disk device. This makes it possible to control an energization amount taking into account thermal expansion of the head element.
Japanese Patent Laid-Open No. 2006-190374 (hereinafter, Patent Document 2) discloses a technique for controlling an energization amount applied to a heater element according to an amount of change in a resistance value of the magnetic resistance effect element. This technique makes use of the fact that the resistance value of a magnetic resistance effect element (a reproduction element) in a head changes according to environmental temperature.
However, with the technique disclosed in Patent Document 1, when a temperature gradient of environmental temperature is steep, a temperature difference occurs between the temperature sensor and the head and the temperature of the head cannot be accurately measured. As a result, a target energization amount cannot be calculated. This is specifically explained with reference to FIG. 19. As shown in FIG. 19, a magnetic disk device 1 disclosed in Patent Document 1 calculates, when environmental temperature is low, a thermal expansion amount of a head element 12c from environmental temperature measured by a thermistor and determines an energization amount applied to a heater element taking into account the calculated thermal expansion amount. As shown in FIG. 19, when the environmental temperature rises, the magnetic disk device 1 determines an energization amount applied to the heater element taking into account a thermal expansion amount of the head element 12c in a high-temperature environment.
Thereafter, as shown in FIG. 19, when the environmental temperature falls, the thermistor measures low temperature. The magnetic disk device 1 determines an energization amount on the basis of a measured value of the thermistor. However, even if the magnetic disk device 1 changes to a low-temperature environment, the temperature of the head element 12c does not immediately fall. In other words, regardless of the fact that the environmental temperature is low, it is likely that the head element 12c will still thermally expand and stay in the high-temperature state for some time. In such a state, if an energization amount the same as that in the low-temperature environment is applied to the heater element, the head element 12c excessively expands and collides with the magnetic disk 11.
The technique disclosed in Patent Document 2 is not suitable for using the magnetic resistance effect element (the reproduction element) as a temperature sensor. Specifically, this is because, when the magnetic resistance effect element is a GMR (Giant Magneto Resistive) element, a resistance value of the magnetic resistance effect element tends to change because of disturbances other than the environmental temperature. This is because the GMR element is formed in multiple layers and has a delicate structure. When the magnetic resistance effect element is a TuMR (Tunneling Magneto Resistive) element, since the TuMR element is formed by an insulating layer, there is almost no change in a resistance value with respect to a change in the environmental temperature. The TuMR element has large individual variation. Moreover, a relation between a resistance change and a temperature change is not linear in the TuMR element. Therefore, regardless of whether the magnetic resistance effect element is the GMR element or the TuMR element, it is impossible to accurately measure the environmental temperature and it is difficult to accurately control an energization amount.
An object of embodiments of the present invention is to solve the problems of the techniques in the past and it is an object of the present invention to provide a head control method, a control device, and a storage device that can accurately control, for respective heads, an energization amount applied to heater elements under any temperature environment and maintain head elements in a target flying height.